This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from my application APPARATUS FOR A MANUFACTURING LONG PERIOD OPTICAL FIBER GRATINGS HAVING LESS DEPENDENCE ON POLARIZATION AND LONG PERIOD OPTICAL FIBER GRATINGS A MANUFACTURED BY THE SAME filed with the Korean Industrial Property Office on Jul. 28, 1999 and there duly assigned Ser. No. 30833/1999.
1. Field of the Invention
The present invention relates to an apparatus for manufacturing long period fiber gratings with low polarization dependence, and long period fiber gratings manufactured by the same.
2. Description of the Related Art
It is known that an induction of perturbations in the refractive index of an optical fiber core using ultraviolet (UV) laser induces birefringence within an optical fiber due to anisotropic perturbations in the refractive index from the viewpoint of the cross-section of the core. For example, A. M. Vengsarkar et al., xe2x80x9cBirefringence reduction in side-written photoinduced fiber devices by a dual exposure methodxe2x80x9d, Opt. Lett. 19, 1260-1262, (1994), concluded that birefringence is due to geometrical asymmetry in respect of the manufacturing conditions for irradiating UV laser to the lateral side of an optical fiber. The refractive index of the core of an optical fiber is generally perturbed along the direction of irradiation of UV light.
Fiber gratings, which are fabricated by perturbations in the refractive index of an optical fiber core due to the irradiation of UV light, also use an asymmetrical irradiation method in certain manufacturing circumstances. Thus, polarization dependency is generated. In particular, the polarization dependency becomes serious in the case of long period fiber gratings which require a refractive index perturbation which is about ten times greater than that for fiber Bragg gratings or short period fiber gratings. This characteristic causes a change in insertion loss due to the polarization of a device, that is, a polarization dependent loss (PDL) or polarization mode dispersion (PMD), so that long period fiber gratings may not be suitable for optical communication elements. In other words, irradiation of UV light onto only one side of an optical fiber induces a birefringence phenomenon in which the optical fiber has different refractive indices along its axis, so that long period fiber gratings have polarization dependency.
Upon optical transmission, the polarization-dependent loss of long period fiber gratings increases as a loss peak increases. Therefore, a long period fiber grating with a large loss peak has a significantly large polarization-dependent loss. In order to reduce the large polarization-dependent loss, a reduction in the polarization dependence of long period fiber gratings is required.
Examples of methods of manufacture of fiber gratings of the conventional art are seen in the following U.S. patents. U.S. Pat. No. 5,502,786, to Inniss et al., entitled METHOD OF FORMING PHOTO-INDUCED DEVICE AND PRODUCT, U.S. Pat. No. 5,528,716, to Inniss et al., entitled METHOD OF DETERMINING AND INVESTIGATING THE PHYSICAL STRUCTURE OF A LIGHT GUIDING BODY, and U.S. Pat. No. 5,559,907, to Inniss et al., entitled METHOD OF CONTROLLING POLARIZATION PROPERTIES OF A PHOTO-INDUCED DEVICE IN AN OPTICAL WAVEGUIDE AND METHOD OF INVESTIGATING STRUCTURE OF AN OPTICAL WAVEGUIDE, describe methods of controlling polarization properties of photoinduced devices. Inniss et al. ""786, for example, describes the fiber optic grating written by exposing one side of the waveguide to light, then exposing the unexposed side to light to impart a desired birefringence.
U.S. Pat. No. 5,625,723, to Dragone et al., entitled METHOD FOR REDUCING BIREFRINGENCE IN OPTICAL GRATINGS, describes a method for reducing birefringence in optical gratings which has a plurality of waveguides, in which the different waveguides are irradiated for different periods of time.
U.S. Pat. No. 5,881,187, to Modavis, entitled OPTICAL WAVEGUIDE FIBER BRAGG GRATING, describes a method for side-writing Bragg gratings with minimal birefringence. In this method, the intrinsic slow axis of the waveguide is oriented relative to the polarization of the light beam to minimize the birefringence.
It is therefore an object of the present invention to provide an improved long period fiber grating.
A further object of the invention is to provide a long period fiber grating with reduced polarization dependent loss.
A yet further object of the invention is to provide a long period fiber grating with reduced variation in refractive index across the fiber.
It is a still further object to provide an improved apparatus and method for manufacturing a long period fiber grating.
Another object of the invention is to provide an apparatus and method which reduce polarization dependent loss in manufactured long period fiber grating.
Accordingly, the present invention provides an apparatus for manufacturing long period fiber gratings with low polarization dependence, in which ultraviolet (UV) light is irradiated after one end of an optical fiber is rotated at least once with respect to the other end thereof.
The present invention also provides an apparatus for manufacturing long period fiber gratings with low polarization dependence, in which UV light is irradiated to an optical fiber, both ends of which are rotated at the same speed.
The present invention also provides an apparatus for manufacturing long period fiber gratings with low polarization dependence, in which UV light is irradiated to an optical fiber, both ends of which are fixed, and UV light which has passed through the optical fiber is axially reflected to be re-irradiated to the optical fiber.
The present invention also provides a long period fiber grating with low polarization dependence, which is manufactured by the manufacturing apparatus.
Specifically, in one embodiment, the present invention provides an apparatus for manufacturing optical fiber gratings, the apparatus including: an optical fiber, one end of which has been rotated at least once with respect to the other end; an ultraviolet laser source; and an amplitude mask disposed over the optical fiber, for transmitting light emitted from the ultraviolet laser source at periodic distance intervals.
In another embodiment, the present invention provides an apparatus for manufacturing fiber gratings, the apparatus including: an optical fiber holder for rotating both ends of an optical fiber at the same speed while supporting both ends of the optical fiber; an ultraviolet laser source; and an amplitude mask disposed over the optical fiber which is being rotated by the optical fiber holder, for transmitting ultraviolet laser light emitted from the ultraviolet laser source to the optical fiber at periodic distance intervals.
In another embodiment, the present invention provides an apparatus for manufacturing long period fiber gratings, the apparatus including: an ultraviolet laser source; an amplitude mask disposed over an optical fiber, for transmitting light emitted from the ultraviolet laser source to the optical fiber at periodic distance intervals; and reflecting bodies installed on the opposite of the optical fiber to the amplitude mask, for reflecting light which has passed through the optical fiber.
In another embodiment, the present invention provides a long period fiber grating manufactured by a method including: twisting an optical fiber at least once; irradiating light to the twisted optical fiber at periodic distance intervals; and untwisting the twisted optical fiber.
The present invention also provides a long period fiber grating manufactured by a method including: rotating both ends of an optical fiber at the same speed; and irradiating light to the optical fiber which is rotating, at periodic distance intervals.